The present invention relates generally to vehicle safety equipment, and, in particular, to safety equipment protecting occupants of a vehicle from injury during a wheels-first crash.
The purpose of occupant restraint systems in motor vehicles is the protection of occupants to increase their survival probability during crash impact of the vehicles. Occupant restraint systems perform three basic functions in order to achieve this: the prevention of occupant ejection from the vehicle, the prevention or minimization of the effects of secondary collisions such as impacts with interior vehicular structures and the control of the crash forces applied to the occupant. Known occupant restraint devices such as three-point lap and shoulder harnesses which perform these functions have been widely researched and improved over the years, thereby significantly increasing automotive safety.
However, attention to the seating portion of automotive restraint systems has not been as extensive. One concern in the area of the seating portion of these systems is that the design of many contemporary automotive seat bottoms have compliant characteristics during vertical loading, +Gz, wherein +Gz is understood to be the upward spinal loading for human occupants of the seats of the type which may occur when an automobile becomes airborne and then lands wheels first. Compliant seating characteristics tend to produce excessive occupant displacements during impulsive loading thereby exposing the occupant to structural strike hazards and offering little management of the crash impact energy.
Hodgson, V. R., Lissner, H. R. and Patrick, I. M., "Response of the Seated Human Cadaver to Acceleration and Jerk With and Without Seat Cushions", The Journal of the Human Factors Society, 1963, disclosed a study with human cadavers during vertical loading. The study concluded that the dynamic load factor, or dynamic overshoot, was increased by all types of cushions used in the tests. The report also concluded that the use of a soft cushion which bottoms during an impulsive force causes more overshoot than the use of no cushion at all. However, soft cushions continue to be used because of perceived occupant comfort in prior art automotive seat cushion technology.
Military technologists have expended considerable effort researching the problem of +Gz exposure, both in the ejection seat and crash-resistant seat areas. This +Gz force vector is an important factor in many occupant survival considerations in military vehicles. For example, helicopter occupants experience significant +Gz acceleration during crashes and hard landings. Ejection seat occupants also experience significant +Gz accelerations as the seat is rapidly propelled from the aircraft.
Military experiments on ejection and helicopter seats have demonstrated that a contoured non-compressible seat pan covered with a rate-dependent foam cushion provides reasonable occupant response by minimizing displacement of the occupant under load. Displacement-controlling seat bottoms define occupant kinematics in a predictable manner. This enables designers to identify potential structural strike hazards and to protect the seat occupant from them. This type of seat bottom prevents occupant response from exhibiting excessive dynamic overshoot.
Human tolerance to +Gz loading may be expressed in terms of both amplitude and time duration. This loading data is commonly reported as Eiband Curves. Using this method of characterization a level of 23 G over approximately a time duration of 5.5 milliseconds has been identified as a critical transition point for the threshold injury region for the human spine. See, for example, Eiband, A. Martin, 1959, Human Tolerance to Rapidly Applied Accelerations: A Summary of the Literature, National Aeronautics and Space Administration, Washington. Subsequent investigation by military researchers in the development of ejection seat designs essentially substantiated the data published by Eiband.
However, it is generally accepted that these tolerance values are based on healthy young males who are ideally positioned with the mid-axillary line of the spine parallel to the acceleration vector. Researchers agree that departure from ideal prepositioning or ideal physiology of the seated occupant tends to lower the ability of the spine to tolerate +Gz loading without serious injury.
Consequently, occupant characteristics such as age, sex, bone strength, and initial position all influence occupant response to impulsive acceleration loading. These factors may increase the likelihood of serious spinal injury even at force levels substantially below the 23 G level set forth by Eiband. As a result of these variables researchers have attempted to define a risk regime of +Gz exposures where extreme caution must be taken to avoid spinal injuries.
Standard military protocol for testing human subjects to +Gz generally permits initial exposure at a safe level of 6 G for training and indoctrination purposes. Acceleration is then gradually increased in 1 G to 2 G increments, using the 8 G through 10 G levels to study kinematic motions. Higher levels, for example, 12 G and beyond, define the risk range. In this protocol, exposure to this risk range is undertaken only after careful analysis of occupant response to accelerations in the safe range.
The close relationship between the safe range and the risk range on the +Gz axis has necessitated careful control of dynamic overshoot within military seating systems in order to prevent avoidable and unnecessary increase of exposure in the risk range. This principle is directly applicable to all vehicles that experience a +Gz acceleration.
Automobile seat designers typically use several different seat bottom design approaches which are very different from the military approach. Some automobile seats contain varied thicknesses of padding material integrated with an array of springs and positioned over an open space within a seat cushion frame. Other designs use thick layers of similar padding material mounted within a rigid seat bottom structure. Still other designs use a hybrid of cushion foams and structures in forming the automotive seat.
It is well known in the field of vehicle safety to provide seat belt restraints for occupants of automobile seats. It is also well known to provide pretensioner devices for eliminating slack in seat belt restraints in order to ideally position occupants including coupling the occupants of the seats with the seat cushions. Conventional pretensioners operate by using well known sensing devices which sense a crash using front crash sensors.
The present invention comprises an automobile safety seat system which has been designed to provide protection from upward force along the spinal column of an occupant of the seat. This is the kind of force applied to an occupant in a wheels-first automobile crash such as a crash into a ditch. The seat bottom of the safety seat system includes a metal automobile seat bucket enclosed on the bottom.
The seat bucket of the present invention is formed to provide a finished undersurface which is covered with an energy absorbing foam. The undersurface includes an overall contour and a definitive anti-submarining ramp for reaction of the buttocks during certain types of collisions. The ramp and underseat construction should be tailored and tuned to each individual automobile station in which they are used. A starting region for such a point design may be the following characteristics: The overall slope angle from front to rear should be a gently contoured angle of approximately fifteen degrees for the first portion of the seat bottom tapering to approximately ten degrees at the back of the seat bottom when completely covered with foam. The ramp structure itself should be a localized angle of approximately thirty degrees assuming a seat front to rear dimension of approximately eighteen inches. A starting location for the ramp may be approximately nine inches from the rear of the seating surface. This would be one of the areas that would need to be specifically tuned for the vehicle.
The seat bucket understructure may be formed of sheet material specifically contoured to the individual application, or it can be made from a generic seat bottom bucket that is filled with a rigid foam insert that is then shaped to the desired contour. For the latter method the seat bucket is filled with a liquid foaming material in order to fill all voids within the seat bucket and to form a bottom cushion which tightly fits within and conforms to the seat bucket. After the liquid foaming material hardens it is either contoured to the buttocks of an idealized occupant and disposed at a ramp angle compatible with the automobile being outfitted or pre-molded to such a shape.
The understructure is then covered with a two-inch layer of environmentally covered energy absorbing foam. The covering layer is a medium density open cell foam which is disposed over any structures in the occupant contact region of the seat in order to further protect the occupant from the covered structures. The covering foam layer is a rate sensitive foam which is easily compressible when force is applied to it slowly and substantially rigid when force is applied to it quickly, for example, during a hard impact. This permits both the comfort required by consumers of automotive products and the advantageous force transmission properties of a more rigid seating surface. Between the fabric and the energy absorbing foam a closed cell comfort pad of a thickness of about one inch may be placed between the rate sensitive foam and the cover. The seat belts of the automobile seat are attached to the seat bottom as close as practical to the intersection between the occupant compressed seat back and the occupant compressed seat bottom in order to control buttocks angulation during unloading events and to aid in keeping the buttocks coupled to the seat. In order for the seat bottom to provide effective dynamic overshoot control in the preferred embodiment of the invention, the seat belt includes a pretensioner. The pretensioner which activates prior to a wheels-first landing applies pre-impact tension to the seat belt in order to prevent slack in the lap region and thereby further improve occupant coupling with the seat cushion. The combination of the belt mounting and the pretensioner help the buttocks of the occupant remain applied to the seat bottom cushion prior to and during the landing of the wheels in the event of a wheel-first crash. The mounting location of the seat belt controls angulation of the belt and ensures that a snug belt maintains the buttocks against the seat cushion when properly angulated. It also helps to maintain the seat belt on the pelvis of the occupant during a variety of other types of crashes. The pretensioner of the prior art responds primarily to a frontal crash sensor. With this invention, the pretensioner also activates when an additional vehicle off-ground sensor indicates that the vehicle containing the inventive seat system is airborne. The vehicle off-ground sensor operates by detecting unweighting of the wheels. Triggering is buffered to prevent inadvertent actuation of the pretensioner during maintenance and lifting of the automobile.